1. Field of the Invention
The present invention relates to an exhaust gas treatment catalyst that remove nitrogen oxides from an exhaust gas discharged from combustion devices such as heavy oil combustion boilers and the like using low-grade fuel and the like and a method for regenerating the exhaust gas treatment catalyst.
2. Description of the Related Art
Recently, focus has been placed on the effective utilization of coal, and in the future, it is predicted that demand on the clean process of using coal will be increased. In order to convert coal into a highly value-added energy medium, advanced technologies, such as a coal gasification technology or a gas purification technology, are used.
As a removal method of nitrogen oxides (NOx) generated from boilers or various combustion furnaces, an ammonia contact reduction method that uses ammonia as a reducing agent and allows a catalyst to decompose nitrogen oxides into nitrogen and water per contiguum has been widely used. As a NOx removal catalyst that has been currently put to practical use, a honeycomb-shaped catalyst having a square aperture shape has become a mainstream in order to prevent blockage by dust in the exhaust gas and increase the contact area of the exhaust gas. Further, as a catalyst component, catalyst components using titanium oxide as a main component are excellent and catalyst components including vanadium, tungsten and the like as an active component are generally used.
In the case of a coal combustion boiler exhaust gas, calcium components contained usually in the dust in the exhaust gas are adhered to the catalyst surface, anhydrous calcium sulfate included in the exhaust gas is produced to cover the catalyst surface, and NO and NH3 gases are prevented from being diffused into the catalyst, thereby degrading the performance of the catalyst. In order to regenerate the catalytic performance of the NOx removal catalysts, there has been proposed a method for recovering the catalytic performance of the NOx removal catalyst, including: cleaning catalyst accumulated materials with an alkali aqueous solution and removing the materials and then performing an activating treatment of the catalyst with an acid aqueous solution (see, for example, Japanese Patent No. 3059136 and Japanese Patent No. 3059137).
Here, several % vanadium (V) is included in the dust in the exhaust gas discharged from heavy oil combustion boilers and the like using low-grade fuel and the like, and vanadium in the exhaust gas is deposited as a V compound such as vanadium oxysulfate (VOSO4) and the like on the surface of the NOx removal catalyst during the operation. Vanadium is a material that is an active component of a NOx removal catalyst and also promotes the oxidation reaction of SO2 as a side reaction, and thus as times passes, the oxidation reaction rate of SO2 in the NOx removal catalyst is increased, thereby increasing the amount of corrosive SO3 discharged into the slipstream side. In addition, when a film of VOSO4 is precipitated on the surface of the NOx removal catalyst, the catalytic performance of the NOx removal catalyst also deteriorates in order to suppress the gas diffusion of the exhaust gas.
However, as a method for recovering the catalyst performance of a nitrification catalyst in the related art, an arsenic compound (As2O3) accumulated on the catalyst surface, which is caused by arsenic (As) included in the exhaust gas discharged from coal combustion boilers, may be efficiently removed, but VOSO4 may not be sufficiently removed. For this reason, when VOSO4 accumulated on the catalyst surface of the NOx removal catalyst is removed to recover the catalytic performance of the NOx removal catalyst and regenerate the NOx removal catalyst, the NOx removal catalyst deteriorates and the strength of the NOx removal catalyst is easily reduced, thereby making it difficult to reuse the NOx removal catalyst.
Thus, in the regeneration of the NOx removal catalyst whose NOx removal performance is reduced by the accumulation of V compounds such as VOSO4 and the like generated by vanadium present in the exhaust gas, there is need for an exhaust gas treatment catalyst which may be regenerated while suppressing the reduction in strength of the NOx removal catalyst and a method for regenerating the exhaust gas treatment catalyst.